Phase binary eutectics

The distribution-coefficient concept is commonly applied to fractional solidification of eutectic systems in the ultrapure portion of the phase diagram. If the quantity of impurity entrapped in the solid phase for whatever reason is proportional to that contained in the melt, then assumption of a constant k is valid. It should be noted that the theoretical yield of a component exhibiting binary eutectic behavior is fixed by the feed composition and position of the eutectic. Also, in contrast to the case of a solid solution, only one component can be obtained in a pure form. 

Figure 2.10. Examples of binary systems characterized by complete mutual solubility in the liquid state and, depending on temperature and/or composition, partial solubility in the solid state and presenting (in certain composition ranges) an invariant (three-phase) reaction (eutectic in the Cu-Ag, peritectic in the Ru-Ni and Re-Co and eutectoidal in Ti-W (one) and in Th-Zr (two)).

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Figure 16.2. Some phase diagrams, (a) The water end of the system potassium chloride and water, (b) The water end of the system sodium chloride and water, (c) The water end of the system magnesium sulfate and water the heptahydrate goes to the mono at 150°C, and to anhydrous at 200°C. (d) /3-methylnaphthalene and /S-chloronaphthalene form solid solutions, (e) Mixtures of formamide and pyridine form a simple eutectic, (f) These mixtures form binary eutectics at the indicated temperatures and a ternary eutectic at mol fractions 0.392 dibenzyl, 0.338 diphenyl, and 0.27 naphthalene.

Naturally, the fixed composition phase transformations treated in this section can be accompanied by local fluctuations in the composition field. Because of the similarity of Fig. 17.3 to a binary eutectic phase diagram, it is apparent that composition plays a similar role to other order parameters, such as molar volume. Before treating the composition order parameter explicitly for a binary alloy, a preliminary distinction between types of order parameters can be obtained. Order parameters such as composition and molar volume are derived from extensive variables any kinetic equations that apply for them must account for any conservation principles that apply to the extensive variable. Order parameters such as the atomic displacement 77 in a piezoelectric transition, or spin in a magnetic transition, are not subject to any conservation principles. Fundamental differences between conserved and nonconserved order parameters are treated in Sections 17.2 and 18.3. 

Figure 12.8 Phase diagram for a binary eutectic system (Gill, 1994).

A three-dimensional model of a ternary phase diagram with a ternary eutectic. T-i corresponds to the AB binary eutectic temperature, T2 to the AC binary eutectic temperature, T3 to the BC binary eutectic temperature, and T4 to the ABC ternary eutectic temperature. 

The second two-liquid field above 1300 °C was assumed to be between a mono-tectic of approximately 55 wt.% S and a composition close to sulfur (which is supercritical at this temperature). The binary eutectic lies at 1350 °C with approximately 67.5 wt.% Cr and 32.5 wt.% S between metallic chromium and the 0-phase,... 

Define alloy, intermetailic compound, phase, variance, eutectic, triple point, binary system. 

Figure 3.20. Phase diagram of the simple binary eutectic system.

The boundary curves, coming out from the individual binary eutectic points, represent curves of the common crystallization of both components of the respective binary subsystems. All boundary curves meet at the ternary eutectic point, which is the lowest temperature where there is the liquid phase in this ternary system. 

The use of the differential thermal analysis in the phase diagram determination is illustrated in Figure 3.52. A hypothetical binary eutectic system A-B with the formation of the incongmently melting compound A4B was chosen. There are three thermograms (a) to (c) shown as examples. In thermogram (a), the first heat effect at temperature ai... 

In such a crystallization process, an extraneous agent is added which leads to creation of a solid phase even before the binary eutectic temperature of the feed components is reached. Here the extraneous agent selectively forms an adduct or an addition compound with one of the two components to be separated. The adduct can be easily separated from the other components and thus both components can be separated in relatively pure forms. 

At about 1300-1340 °C (binary eutectic temperature WC-Co 1310 °C ternary eutectic temperature W-Co-C 1275/80 °C) partial melting occurs, and more WC is dissolved until the eutectic concentration (54% Co and 46% WC) is reached [9.8]. A further increase in temperature results in additional dissolution of WC and complete melting of the binder phase. In this stage, rapid final densification occurs and the sintered body is practically pore-free. 

The volume fraction of reinforced phase in eutectics is 7.7 % and 31-wol.% for systems Ti-B and Ti-Si, respectively. The typical structures of eutectic alloys for Ti-Si system is shown in Fig. 2. According to binary diagrams of phase equlibria, an essential solubility of silicon in a- and 13-phases is observed, which is dependent on temperature there is an eutectoid transformation (in this respect diagram Ti-Si is similar to Fe-C diagram), but in system Ti-B essential solubility of boron in a- and 3- phases does not occur. For this reason the structure of composites of Ti-B system is more stable at temperature variation. 

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